This invention relates to drying of wood veneer, more particularly to drying of wood veneer by applying high frequency energy such as microwave energy
Drying of veneer for laminating purposes including the formation of lamented veneer lumber (LVL) or products made by combining long strips of veneer under heat and pressure to form a relatively large cross section billet which may be later processed to form relatively strong lumber and/or beams such as those sold under the trademark xe2x80x9cParallamxe2x80x9d by Weyerhaeuser Company, generally comprises drying the veneer one sheet at a time. Normally the veneer is dried by hot air jet drying i.e. blasting hot air against opposite side faces of the veneer. This requires a relatively long drier to provide sufficient time to complete the drying.
The veneer is dried to a low moisture content before it is made into a finished product, typically to between about 0.1% and 10% moisture on a weight of day wood basis. If the veneer dryness is outside of this narrow range the quality of the final product will likely be negatively impacted. Both too dry and too wet veneers have a negative impact.
Veneer sheets to be dried usually contain significantly different moisture contents both between different veneer sheet and locally within a single veneer sheet, and further generally do not have constant density throughout their areas, which makes uniform drying more difficult. Thus although the treatment is the same for all sheets it is not uncommon for the final moisture content in the dried veneer sheets to vary significantly both from sheet to sheet and internally (locally) within a given sheet and generally the sheets are separated based on wetness and the wetter sheets subjected to a redrying stage where the dried veneer still containing significant moisture (e.g. wet spots in the veneer) are redried.
Redrying may be done for example by passing the wet sheets through the same drier a second time and choosing a time and temperature to gain the desired result, but again the wetness of the individual sheets vary and it is difficult to set conditions to properly process these wet sheets.
It is also known to use radio frequency drying of a batch (pile of veneers) typically a 4 by 8 by 3 foot high stack of veneers (120 by 240 by 90 cm) or pass the sheets singly through a radio frequency drier, but both of these techniques while operative are not particularly effective
It is known to apply high frequency energy such as microwave energy to heat or dry materials. For example in the above-described processes of making xe2x80x9cParallamxe2x80x9d microwave energy is used and it is also sometimes used in making LVL.
In the manufacture of plywood and LVL it is the practice to offset the end of the veneer in adjacent layers to better insure there is no relatively weak section formed by a plurality of layer having the edges of the veneer in various layers aligned or close being aligned vertically through the finished product see fore example U.S. Pat. Nos. 3,963,552 issued Jun. 15, 1976 to Troutner et al. and 4,797,169 issued Jan. 10, 1989 to Alzawa et al.
It is an object of the invention to provide a veneer drying system to more uniformly dry the veneer.
It is a further object of the present invention to provide a more efficient drying system for veneer, which permits easy separation for individual testing of each veneer sheet or stacking for further use.
Broadly the presenting invention relates to a method of drying veneer sheets comprising, laying each succeeding single veneer sheet into overlying relation with its immediately preceding veneer sheet to form a layup of shingled veneer sheets with a said immediately preceding veneer sheet of said layup leading its said succeeding veneer sheet by a distance X of at least 1 inch (2.5 cm) and not more than 50% of the length of one of said veneer sheet, continuously feeding said layup into a drier in a direction substantially the same as the direction said immediately preceding veneer sheet leads its said succeeding veneer sheet, drying in said drier said veneer sheets forming said layup to provide dried veneer sheets, separating sequentially said dried preceding veneer sheets from their dried succeeding veneer sheets in said layup into discreet dried veneer sheets.
Preferably the drying stage will apply high frequency drying.
Preferably said high frequency drying will be microwave drying.
Preferably said high frequency drying will be radio frequency drying.
Preferably said veneer sheet have their grain direction substantially parallel to said direction.
Preferably said distance X is 1 foot (30 cm).
Preferably said laying and said feeding are coordinated so that said layup is between 5 and 25 veneer thicknesses as it moves through said drier and is dried.
Preferably said veneer sheets have faces with surface areas of at least 7 square feet (6300 square cm) and a thickness of between {fraction (1/16)} and {fraction (3/16)} inches (0.15 and 0.3 cm).
Preferably said separating comprises pinching said layup between a pair of bottom and top pinch rolls and continuing to move said layup in said direction by said pinch rolls conveying a lowermost veneer from said layup immediately after it passes off said bottom roll at a speed sufficient to singulate said lowermost veneer from said veneer remaining in said layup.